1. Field of the Invention
The present invention relates to lasers and, more specifically, to apparatus for obtaining high power pulses from gas lasers.
2. Description of the Technology
Laser rangefinder equipment is used to determine the distance from a rangefinder to a remote object. In order to maximize the resolving power of a laser rangefinder, such a system must be capable of generating pulses which contain relatively high levels of energy but which span only an extremely short interval of time. A desirable peak power level for such a system is in the 1 megawatt range. The total energy transmitted in one of these pulses should be in the neighborhood of 100 millijoules. The duration of such a pulse, measured at the one-half of peak amplitude power level, is approximately 60 nanoseconds. For optimum performance of rangefinders which employ shared aperature between the transmitter and the receiver, the total length of the pulses must be less than a microsecond i.e., in one microsecond or less from the occurrence of the peak, the power must decrease by a factor equal to 10.sup.-6.
The design constraints of weight, size, and volume further complicate the criteria which dictate the construction of the system. Such design constraints often require the active volume of such a laser rangefinder to be approximately from 15-20 cubic centimeters. CO.sub.2 laser rangefinder which are currently available can typically produce pulses of 35-50 millijoules of energy over a time span of 2 microseconds. This 2 microsecond pulse length causes severe difficulties by degrading a rangefinder's capacity to accurately resolve distances.
The origin of this inability of a rangefinder to resolve pulses is related to the lack of sharpness of the shape of the output waveform. In comparison to the ideal high, narrow pulse shown in FIG. 1, FIG. 2 depicts an output pulse which contains less energy and is far less sharp or spiked. This lack of pulse definition often results in the inability of a rangefinder to distinguish between two closely separated targets. The secondary peaks which accompany the primary peak in the waveform shown in FIG. 2 makes such waveforms unsuitable for use in laser rangefinders.
One technique which has been employed to obtain higher power and sharper pulses is to increase the gas pressure of the laser gain medium. This method is used in a transverse electric laser in which the excitation energy is imposed upon a gas medium in a direction perpendicular to the optical axis of the laser cavity. Instead of maintaining a CO.sub.2 mixture of carbon dioxide nitrogen and helium at 1 atmosphere, gas lasers may be operated at several multiples of 1 atmosphere in order to produce high energy and short time duration laser pulses.
One attempt to produce high output pulses is described in U.S. Pat. No. 4,185,255-Wittman, et al. That apparatus includes a pair of electrodes disposed in parallel fashion around a gas containing laser tube. A rather large generator unit is coupled to these electrodes and provides large amounts of energy in order to trigger the gas laser and thereby produce relatively high output pulses of several kilojoules. The problem with this apparatus is that it is incapable of producing output pulses of great magnitude which are required in order to produce a state of the art rangefinder. If the gas pressure within such a device is greatly increased, so that the CO.sub.2 mixture is contained in the range of about 5 atmospheres, then the device is capable of producing the required power output. One problem created by increased operating gas pressure is unwanted arcing of electrical energy across the discharge electrodes which severly degrades the laser output energy. It is possible to decrease the probability of arcing by decreasing the inductance of the circuit which, in turn, reduces the electrical pulse width.
Another technique employed to produce more intense output pulses would be to use an electro-optic Q-switch. Such a method incorporates a suitable crystal, e.g., cadmium telluride, in order to control the laser pulse shape and energy. This method requires a great deal of additional equipment and a separate power supply, and is susceptible to crystal damage and misalignment caused by rough use.
It would be highly advantageous to develop an integrated laser head which is capable of generating extremely short, high energy output pulses. Such a solution would satisfy a long felt need manifested by the current efforts of the laser and optics industry which continues to develop communications and measurement systems which require reliable, durable, cost effective high output lasers. The continued development and manufacture of such high power output lasers has generated a concomitant demand for an invention suitable for use in a compact laser rangefinder which can produce 100 millijoules output pulses which have a pulse length of less than 1 microsecond. None of the prior devices provides an effective and inexpensive solution to this problem which is encountered in the rangefinder technology. Such an integrated laser head would ideally be suited to operate within a very small working volume, typically 15-20 cubic centimeters, so that it would be capable of being used in a variety of situations and environments. Such an integrated laser head would further be capable of being employed in a wide variety of military and civilian uses over a broad range of temperatures and pulse output rates.